1. Field of the Invention
The present invention relates to a configuration for a handheld vector network analyzer (VNA) to enable improved operation in the presence of external signals.
2. Description of the Related Art
Wireless communications networks such as cellular telephones or personal communication services (PCS) telephones require remotely located antennas or xe2x80x9ccellsxe2x80x9d to provide services to individual users. To reduce service costs for the remote cell site, it is desirable to test the microwave components of the cell site at its remote location and locate and repair portions which fail.
A VNA can be used to test for component malfunctions at cell sites. As illustrated in FIG. 1, a VNA operates by sending out a signal on its test port and then measuring the reflected signal on the same port. The incident and reflected signals are then downconverted, and a magnitude and phase of the reflected signal relative to the incident signal are displayed as measurement results.
VNAs enable a user to easily identify a fault in a cell site component and to measure the distance from the test device to a fault or discontinuity. A fault may result from environmental conditions such as corrosion of a connector, or from faulty installation or repair. Test measurements are first typically made using a VNA operating in the frequency domain to determine whether a fault exists as indicated by an undesirable standing wave ratio (SWR). If a fault is discovered, an analysis of the results derived from the frequency domain measurement is made in the time domain to locate the position of the fault.
A VNA also enables calibration to extend the test port connection to the end of a cable connected to the test port by accounting for any phase and amplitude differences. By using a VNA to extend the test port to the end of the cable, errors in the cable will not be taken into account when measuring a device through the cable.
Because a remote cell site may be constantly providing signals for communications devices once it is installed, it is desirable that the cell site not be disabled for testing. By measuring both phase and amplitude, a VNA can include components to enable extraneous signals to be distinguished and their affects minimized, enabling tests to be performed with the remote cell site active.
Many VNAs are large and not easily transportable. For instance, VNAs such as the HP 8510 manufactured by Hewlett Packard, have a housing greater than two feet on a side and a power supply weighing 50 pounds or more. Such VNAs are difficult to transport to remote facilities where cell cites for cellular telephones and PCS telephones are located.
More recently Anritsu Company of Morgan Hill Calif. has manufactured a Sitemaster VNA, which is a handheld device with dimensions on the order of 8 inches by 6 inches by 2xc2xd inches, which weighs on the order of 3 pounds. One version of the Sitemaster has components described in U.S. Pat. No. 5,642,039 entitled xe2x80x9cHandheld Vector Network Analyzerxe2x80x9d, which is incorporated herein by reference. Other versions of the Sitemaster VNA are described in the following U.S. Patent Applications: U.S. patent application Ser. No. 08/865,882, entitled Two Port Handheld Vector Network Analyzer With Frequency Monitor Modexe2x80x9d, filed May 30, 1997 and incorporated herein by reference; and U.S. patent application Ser. No. 08/957,261, entitled xe2x80x9cHandheld Vector Network Analyzer (VNA) Operating At A High Frequency By Mixing LO and RF Signals Having Offset Odd Harmonicsxe2x80x9d, filed Oct. 4, 1997 and incorporated herein by reference.
With a limited size and weight, Sitemaster VNAs provide a VNA which is easily transported and used at remote cell cites. Embodiments of the Sitemaster VNA, as described in U.S. Pat. No. 5,642,039 referenced above, include specialized analog components to reduce interference created by signals external to the system being tested.
In accordance with the present invention, a VNA measurement method is disclosed enabling increased immunity to interference created when the VNA is operated in the presence of external signals.
In accordance with the measurement method of the present invention, three measurements are first taken, one at a desired measurement frequency, another at a frequency slightly less than the desired measurement frequency, and another at a slightly greater frequency than the desired measurement frequency. An external signal at a particular frequency will interfere to a greater extent with one of the three measurements ag opposed to the other two. To eliminate measurement error resulting from the interfering signal, the measurement signal with the median, or middle, magnitude is selected to provide the measurement results.
The method of the present invention can be used alone or in conjunction with the analog system described in U.S. Pat. No. 5,642,039 to help reduce interference created by external signals.